Recently, it is desirable to reduce the size of wiring patterns of wiring boards to micro wiring patterns, in order to cope with demands to reduce the size of electronic devices and to increase signal transmission rates or speeds of the electronic devices. For this reason, various kinds of structures have been proposed for the wiring boards having the micro wiring patterns. For example, Japanese Laid-Open Patent Publication No. 2010-87508 proposes one example of the wiring board having the micro wiring patterns.
On the other hand, crosstalk noise occurs when the size of the wiring pattern is reduced to the micro wiring pattern. Hence, as a measure against the crosstalk noise, structures including a microstrip structure in which a planar layer is provided above or below a wiring pattern, a coplanar structure in which a shield pattern, such as ground or the like, is inserted between wiring patterns, or the like have been studied.
However, according to the microstrip structure or the coplanar structure, the number of layers of the wiring board increases to make the wiring board thick, because the planar layer is inserted. In addition, in the case of the coplanar structure, a density of signal wirings decreases because of the shield pattern that is inserted between the wiring patterns, and as a result, the number of layers of the wiring board increases to make the wiring board thick.